This invention relates to a radar receiver capable of discriminating between fixed frequency and variable frequency signals in the X- and K-band portions of the radio spectrum.
This invention is particularly useful in connection with a receiver designed to detect signals emitted by police speed radar detectors, and to distinguish those fixed frequency signals from other signals emitted by nearby superheterodyne-type radar detectors.
Some newly manufactured radar receivers have been introduced to the marketplace using the so-called superhomodyne scheme for detecting frequencies in the X- and K-bands. These receivers use a signal generator or local oscillator on the same frequency as the signal to be received, and this internal signal tends to be re-emitted by the antenna of the receiver. Although its power level is low, the proximity of a receiver of this type to a sensitive receiver could make it appear that a radar transmitter is in the vicinity, thus sounding an alarm. Since the local oscillator in a superhomodyne receiver is at the same frequency as the received signal, it is impossible to trap that signal and thus prevent it from being reradiated by the antenna.
Thus, receivers of the superhomodyne type are continuously broadcasting X- and K-band signals. They do have one characteristic, however, that permits their signals to be distinguished from police radar signals, and that is the frequency of the superhomodyne emitted signal is constantly varying over the range of frequencies that it is designed to detect.
Accordingly, this invention provides means for distinguishing between the fixed frequency emission of a police radar transmitter and the varable frequency emissions from a superhomodyne-type receiver.